This invention relates to electrophotography, and more particularly to novel photoconductive compositions which provide a reduced image contrast.
The process of xerography, as disclosed by Carlson, for example, in U.S. Pat. No. 2,297,691, employs an electrophotographic or photoconductive element comprising a coating of a photoconductive insulating material on a conductive support. The photoconductive element is given a uniform surface charge in the dark and is then exposed to an image pattern of activating electromagnetic radiation such as light or x-rays. The charge on the photoconductive element is dissipated in the illuminated area to form an electrostatic charge pattern which is then developed by contact with electroscopic marking material. The marking material or toner, as it is also called, whether carried in an insulating liquid or in the form of a dry powder, deposits on the exposed surface with either the charge pattern or the discharge pattern, as desired. Then, if the photoconductive element is of the non-reusable type, the developed image is fixed by fusion or otherwise to the surface of the photoconductive element. If the element is of the reusable type, e.g., a selenium-coated drum, the image is transferred to another surface such as paper and then fixed to provide a copy of the original.
A problem that has existed is the control of electrostatic contrast in the electrophotographic materials. Electrostatic contrast can be defined quantitatively in terms of gamma (.gamma.), which is derived from the electrical H and D curve. This curve is analogous to the type of curve first employed by Hurter and Driffield for silver halide sensitometry, except that voltage or charge on the electrophotographic element is used instead of silver density. In the straight portion of the electrical H and D curve, where the change in voltage (which corresponds to the change in image density) is proportional to the change in the logarithm of the exposure, the slope is called gamma (.gamma.) and provides a quantitative measure of the electrographic contrast.
It is recognized in the art that p-type organic photoconductive materials (that is, these organic photoconductors whose primary mode of photoconduction is provided by positive charge carriers, i.e., holes), can be sensitized or activated by dyestuffs or other materials which serve as electron-trapping agents. For example, U.S. Pat. No. 3,287,123 of Hoegl at column 2, lines 20-42 describes several broad classes of organic photoconductors (or "electron donors" in the nomenclature of the Hoegl patent) and further discloses that such p-type photoconductors can be sensitized by "activator" substances which act as electron-acceptors. An electron-acceptor is a material which (as is apparent from the term "electron-acceptor") is an electron-trapping agent. Similarly, U.S. Pat. No. 3,745,160 of Daniel et al. discloses a novel class of spectral sensitizing dyes particularly useful for p-type organic photoconductors. The class of sensitizing dyes disclosed in Daniel et al are cyanine dyes containing a 1,3,2-dioxazaborinium salt or a 1,3,2-diazaborinium salt. In general, the dioxazaborinium and diazaborinium sensitizing dye salts disclosed therein are materials which desensitize negative silver bromoiodide emulsions, and therefore these materials also are electron-trapping agents (column 2, lines 41-53). This is because materials which desensitize negative silver bromoiodide emulsions generally do so by trapping electrons generated by the light sensitive silver bromoiodide grains thereof. Likewise, the various sensitizing materials for p-type organic photoconductors described in U.S. Pat. No. 3,814,600 of Contois at Tables I and II, column 6, lines 15-18, and column 7, lines 5-29, provide sensitization of p-type organic photoconductive materials by acting as electron-trapping agents. This is specifically noted for certain of the dye salt sensitizers disclosed in the Contois patent, for example, see column 5, lines 29-42 thereof. Similarly, U.S. Pat. No. 3,769,011 of Gilman et al. describes a variety of J-aggregated methine dye salts which can be used to sensitize p-type organic photoconductors (See column 8, lines 17-52). In the case of Gilman et al. the J-aggregated methine dyes serve as electron-trapping agents and thereby provide sensitization of the p-type organic photoconductive compositions.
Much of the research and development work which has been carried out with respect to the sensitization of p-type organic photoconductive compositions has been directed, as indicated in the above-noted patents, to the discovery of new and improved sensitizing addenda for these photoconductors, for example, sensitizing dyes such as dye cations of dye salts. For this reason, until the present invention, it was not appreciated in the electrophotographic art that, if a particular sensitizing dye consisted of a dye salt, the anion of the dye salt could also have an important effect on the resultant properties of the organic photoconductive composition.
Typically, p-type organic photoconductive materials such as those described in the above-noted prior art patents, i.e., those compositions which incorporate various electron trapping agents as sensitizers, tend to exhibit relatively high contrast. In some applications of electrophotographic materials, this increase in contrast is unimportant. In other applications, however, it is desired that contrast be decreased or minimized. Many compounds known to control contrast, however, result in decreased sensitization. Therefore, there is a need in the art for a method of controlling contrast in electrophotographic materials without substantial reduction in electrical speed or other desired qualities.
Further, it is also desired that contrast controlling compounds be soluble, i.e. non-crystalline, in solid solutions of photoconductive materials.
It is an object of this invention to provide photoconductive compositions capable of producing electrophotographic reproductions of reduced contrast, preferably having a relative average contrast at least 10% lower than the same composition without the contrast controlling material.
It is a further object of this invention to provide an electrophotographic element which has a particular desired contrast. For example, it may be desired initially to employ an element which has a relatively high contrast and then to make copies of the high contrast image with a photoconductive element of lower contrast.
It is a further object of this invention to develop methods for producing electrophotographic elements from said photoconductive compositions.